Femtocell base stations are known in different communication standards. For instance, the 3GPP (3rd Generation Partnership Project) standard with specification number 3GPP TS 25.467 v 10.2.0 describes the UTRAN (Universal Terrestrial Radio Access Network) architecture for femtocell for 3G (3rd Generation) Home Node B (HNB).
Specifically, the 3GPP TS 25.467 standard specification describes a UE (User Equipment) registration function as “The UE Registration Function for HNB provides means for the HNB to convey UE identification data to the HNB GW in order to perform access control or membership verification for the UE in the HNB GW. The UE Registration also informs the HNB GW of the specific HNB where the UE is located.”
According to the standard, UE registration is performed by the HNB and the HNB registers a specific UE with the HNB GW. The registration is triggered when the UE attempts to access the HNB via an initial NAS (Non-Access Stratum) message over the Uu interface (e.g., Location Updating Request) and there is no context in the HNB allocated for that UE.
Chapter 5.1.2 of the same standard describes the case where a HNB registers a UE in the HNB GW when either the UE or the HNB, or both, do not belong to a CSG (Closed Subscriber Group).A CSG is a list of UEs which are allowed to have access to the HNB. The list can be maintained either in the HNB or HNB GW or in the Core Network (CN), and it may comprise for example the CSG Identity (CSG-ID) of the CSG and the IMSIs (International Mobile Subscriber Identities) of the UEs allowed to access the CSG. In this case, the standard describes the option that the HNB may in certain cases fetch the identity of the subscriber with a NAS Identity Request procedure before registering the subscriber in the HNB GW. This step consists of a NAS Identity Request message sent by the HNB, and a NAS Identity Response message sent by the UE as a reply, containing the subscriber identity (IMSI).
A problem with the existing standard as described above is that the HNB uses a NAS message to fetch the identity of the subscriber. However, NAS messages are defined to be exchanged between the CN (Core Network) and the UE (User Equipment), and they are standardized in 3GPP TS 24.008.
The NAS messages contain a sequence number, N(SD), that allows the CN to detect duplicated NAS messages sent by the UE. NAS messages may be duplicated by the Data Link layer in some cases, for example at channel change before the last layer 2 frame has been acknowledged by the peer. The NAS sequence number mechanism is described in 3GPP TS 24.007, chapter 11.2.3.2.3 and subchapters. The first NAS message is sent by the UE with sequence number ‘0’. The CN will receive this first NAS message with sequence number ‘0’ in an RANAP (Radio Access Network Application Part) Initial UE message. However, when the HNB fetches the subscriber identity with a NAS Identity Request message, this will cause the UE to increment the sequence number and reply with a NAS Identity Response with sequence number ‘1’. This will result in the CN discarding further NAS messages sent by the UE to the CN, since the next NAS message to the CN will contain the sequence number ‘2’, but the CN expects the next NAS message to have the sequence number ‘1’ according to TS 24.007. Thus, the Location Update in the CN will fail in certain cases, for example if the CN tries to authenticate the subscriber with a NAS Authentication Request, since the CN will discard the Authentication Response from the UE.